The present invention relates to sealable multilayer films of unusually high opacity and unusually low light transmission and to a method of making said films.
In the packaging of certain types of foods, such as snack foods like potato chips, cookies and the like, it is common practice to employ a multilayer film. A desirable property in such a packaging film is an opacity which protects the packaging product from deterioration caused by exposure to light. In particular, it has been found that certain wavelengths of light, up to about 450 nm cause increased spoilage in such packaged products. Even when a degree of opacity is present in the film, spoilage occurs if the film allows passage of some light.
Oriented opaque film compositions are known in the art. U.S. Pat. No. 4,377,616 discloses an opaque biaxially oriented polymer film structure comprising a thermoplastic polymer matrix core possessing numerous voids, a substantial number of which contain at least one spherical void-initiating particle, and transparent thermoplastic skin layers adhering to the surfaces of the core layer.
U.S. Pat. No. 4,632,869 discloses an opaque, biaxially oriented film structure having a polymer matrix with a strata of voids, the voids containing spherical void-initiating particles of polybutylene terephthalate. The structure may also include thermoplastic skin layers and the film can include from about 1% to 3% by weight of a pigment such as TiO.sub.2 or colored oxides.
U.S. Pat. No. 4,758,462 also discloses an opaque, biaxially oriented film with a cavitated core and transparent skin layers. Colored light absorbing pigments such as carbon black or iron oxide are added to the core and/or the skins in an amount of about 2 to 12 weight % to decrease light transmission through the film.
U.S. Pat. No. 4,652,489 discloses an oriented, sealable, opaque polyolefin multilayer film with a core layer containing vacuoles, a sealable surface layer, and a non-sealable surface layer which incorporates a slip agent such as a polydiorganosiloxane.
U.S. Pat. No. 4,741,950 discloses a differential opaque polymer film with a core layer containing numerous microscopic voids, a rough-appearing wettable first skin layer which contains an antiblocking agent such as silica, silicate, clay, diatomaceous earth, talc and glass, and a second wettable skin layer with a smooth appearance which may be metallized. TiO.sub.2 may be present in the core and/or first skin layer. The film allows a light transmission of 24%.
All of the aforementioned films are opaque, but none provides a film with essentially 100% opacity with exceptionally low light transmission, especially in the damaging UV and blue wavelengths. Nor do these references disclose an economical asymmetrical multilayer film structure with differential appearance having metallic appearance on one side and a substantially whiter opposite side which allows printing or bonding to a substrate or coating.
Accordingly, it is an object of the present invention to provide a film with exceptionally low light transmission especially in the UV and blue wavelengths.
It is a further object of the present invention to provide a film which also has high opacity.
It is another object of the present invention to provide a film with a highly functional differential appearance.
It is yet another object of the present invention to provide a film which may be bonded to a wide variety of substrates and coatings.